Conventionally, in an optical circuit; light output from an optical waveguide falls onto a mirror tilted at 45 degrees and is reflected by the mirror (see, e.g., Japanese Laid-Open Patent Publication No. 2006-201508). Further, a conventional optical signal input device has a structure such that the optical axis of a condensing lens that condenses parallel light output from a collimator lens onto the entrance of an optical waveguide is shifted relative to the optical axis of the collimator lens (see, e.g., Japanese Laid-Open. Patent Publication No. 2006-235115).
To suppress loss in an optical coupling structure that causes light reflected by a mirror tilted at 45 degrees to fail, onto an optical waveguide, it is necessary to cause the light to be reflected by the mirror surface and to cause the light entering the optical waveguide to be completely reflected by an interface between the core and the cladding of the optical waveguide. However, even if the incident position or angle of light is not shifted relative to the mirror surface, transmission loss consequent to light passing through the mirror inevitably results.
According to a trial calculation by a photoelectromagnetic field analysis based on the finite difference time domain (FDTD) method, such optical transmission loss is calculated at approximately 0.3 dB. This analysis value is given on the condition that a core-cladding specific refractive index difference Δ is set to about 1.9% and the numerical aperture (NA) of a light source is set to 0.2. Elements other than the mirror also cause loss, and the loss caused by the mirror must be reduced to almost zero in an ultra-high-frequency band in which, for example, the optical transmission speed is 40 Gbps or higher. With consideration these factors, the suppression of transmission loss caused by the mirror arises as a problem to foe addressed.